1. Field of the Invention
The present invention relates to a signal bandwidth extension apparatus which converts a band-limited signal such as a speech signal, music signal, or audio signal into a wideband signal.
2. Description of the Related Art
As is well known, upon extending the bandwidth of a signal such as a speech signal, music signal, or audio signal (input signal) to a wideband signal, a bandwidth-extended signal (output signal) in a voiced sound has to maintain a structure (harmonic structure) in which a fundamental frequency and its overtones have peaks in a frequency domain and many components are present at frequency intervals of the fundamental frequency, so that the extended signal sounds like a natural sound in place of an artificial sound. Conventionally, the bandwidth extension method is roughly classified into a first method for generating a harmonic structure by extracting the fundamental frequency (for example, Jpn. Pat. Appln. KOKAI Publication No. 9-55778) and a second method for generating a harmonic structure by, e.g., nonlinear processing without extracting any fundamental frequency (for example, the Acoustical Society of Japan Transactions (October, 1994) “Telephone speech Enhancement by Bandwidth Expansion and Spectral Equalization”, 1-P-6, pp. 349-350 (Fujitsu Laboratories Ltd.)).
The first method applies linear prediction analysis to an input signal to extract a fundamental frequency. Then, a linear prediction residual signal (excitation signal) is frequency-shifted by integer multiples of the fundamental frequency. The shifted signal is synthesized by a linear prediction synthesis filter, thus obtaining a bandwidth-extended signal. However, with this method, a heavy computational load is required to extract the fundamental frequency. Also, since there is no reliable extraction method of the fundamental frequency, unstable fundamental frequency extraction precision largely influences the overall sound quality.
On the other hand, the second method associated with the Acoustical Society of Japan Transactions (October, 1994) “Telephone speech Enhancement by Bandwidth Expansion and Spectral Equalization”, 1-P-6, pp. 349-350 (Fujitsu Laboratories Ltd.) applies linear prediction analysis to an input signal, and applies nonlinear processing based on half-wave rectification to a linear prediction residual signal to extend a low-frequency bandwidth. Furthermore, a low-frequency bandwidth-extended signal is obtained by synthesis of a linear prediction synthesis filter. With this second method, although the computational load is light, a prediction signal which is not included in an actual sound (original sound) is generated, resulting in poor sound quality.
The conventional signal bandwidth extension apparatus requires a heavy computational load to extract the fundamental frequency or generates a prediction signal which is not included in an original sound, resulting in poor sound quality.